Implement Elder Ray Strategy

[cinar]

Implement the Elder Ray trading strategy using the existing momentum.ElderRay indicator.

Location: strategy/momentum/elder_ray_strategy.go

Strategy Rules:

• Buy Action: EMA(13) is rising AND Bear Power is negative but rising.
• Sell Action: EMA(13) is falling AND Bull Power is positive but falling.
• Hold Action: Otherwise.

Implementation Details:

• Follow the strategy.Strategy interface.
• Use momentum.NewElderRay() or allow custom parameters.
• Provide unit tests with historical data in strategy/momentum/testdata/elder_ray_strategy.csv.
• Ensure 100% test coverage.